net: phy: add constants for 2.5G and 5G speed in PCS speed register

Add constants for 2.5G and 5G speed in PCS speed register into mdio.h.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>